1500 PHOTOCHEMISTRY OF CHLOROPHYLL CHAP. 35 



of ferrous acetate on pheophytin in acetic acid and found that in ethanol this compound 

 is oxidized by air (presumably to a ferric complex) with a band shift from 645 to 610 m/x; 

 upon illumination the band is shifted back to 645 m^ (presumably indicating reduction 

 to the ferrous form). 



The chlorophyll-ferric ion reaction and the possible formation of metal 

 compounds of chlorophyll (with magnesium still in the center of the mole- 

 cule and the metal ion attached elsewhere— perhaps to the enol group in 

 ring V) remains in need of further study. As mentioned on pages 465 and 

 492, the similarity between the reaction of chlorophyll with Fe+++, and the 

 first, reversible stage of the "phase test" is suggestive; the first (reversible 

 and light-sensitive) step in the conversion of chlorophyll to pheophytin 

 (cf. page 493) also bears an outward similarity to these reactions. 



Reversible photobleaching (probably photoxidation) of chlorophyll by 

 quinones was described by Linschitz and Rennert (1952). To block the 

 thermal back reaction they carried out the irradiation at - 190° in glassy 

 solvent (8 parts ether -j- 3 parts isopentane + 5 parts ethanol, by volume). 

 The bleaching effect in red and blue, and the enhancement of absorption 

 in green (480-590 mM) and far red (>700 m^), noted upon illumination of 

 the oxygen-free chlorophyll solution {cf. above, section 1) is strongly en- 

 hanced by the addition of 10 -^ or 10"* mole/1, of a quinone or imine; the 

 bleached state survives in this case in the dark until the solvent is melted. 



Evstigneev, Gavrilova and Krasnovsky (1950) measured the quenching 

 of chlorophyll fluorescence by various organic reagents {cf. table 23.IIIC, 

 and chapter 37C, section 4c) in pyridine and ethanol, and their influence 

 on the red absorption peak of chlorophyll {cf. chapter 21, page 647 and 

 chapter 37C, section 2). They inquired whether these effects occur to- 

 gether, and whether they are associated with accelerated photochemical 

 bleaching of chlorophyll, but found no correlation. Thus, quinone, which 

 is the strongest quencher of chlorophyll fluorescence in both ethanol and 

 pyridine, has no effect on absorption and causes no photochemical bleaching 

 in ethanol (and only a relatively slow one in pyridine). Ascorbic acid, 

 which produces the fastest photochemical bleaching in pyridine (and is 

 second only to oxygen in ethanol), has no effect on either fluorescence or 

 absorption. Evstigneev et al. looked not only for the easily detectable 

 progressive (slowly reversible or irreversible) photochemical bleaching of 

 chlorophyll by quinone, but also for the more elusive rapidly reversible, 

 stationary bleaching during the illumination (by sending a scanning beam 

 through a strongly illuminated solution) ; but found no observable effect 

 (compare, however, the above-described low temperature observations of 

 Linschitz with the same system). They concluded that the reaction which 

 leads to bleaching occurs not to the short lived, fluorescent chlorophyll 

 molecules, but to the long lived, metastable ones (tChl), and therefore has 



